HVDC transmission

ABSTRACT

An HVDC transmission including at least two converters. Each converter is adapted for connection between a three-phase alternating-voltage network. At least a first converter has ac leads for connection of the first converter to the alternating-voltage network without the use of a separate winding transformer. A dc link is common to the converters. A zero-sequence inductor connection is arranged in the ac leads of the first converter and designed such that the zero-sequence inductor connection exhibits a high impedance to zero-sequence currents and a low impedance to positive-sequence currents.

FIELD OF THE INVENTION

The present invention relates to an HVDC transmission with at least twoconverters. Each converter is adapted for connection between athree-phase alternating voltage network and a dc link common to theconverters. At least a first converter has ac leads for connection ofthe converter to its alternating-voltage network without the use of aseparate winding transformer, or "full transformer".

The concept "HVDC transmission" means in this application an electricinstallation or equipment for power transmission by means ofhigh-voltage direct current. The concept comprises two main types ofinstallations. The first of these types consists of installationsadapted for power transmission between two or more spaced-apartconverter stations, which are interconnected by means of dc-carryingcables or overhead lines. The second of the types consists of so-calledback-to-back connections, in which two converters arranged in the sameconverter station are each connected to a separate alternating-voltagenetwork, are dc-connected to each other and adapted for controllablepower transmission between the alternating-voltage networks.

In the first type of installation, the dc link consists of the cable orcables or lines which connect the dc sides of the converter stations. Inthe second type of installation, the dc link generally consists only ofa pair of busbars in the station. In both cases, however, certaindevices for smoothing and filtering of the direct current, for currentand voltage measurement, for protection against overvoltages, amongother functions included in the dc link in a known manner.

The expression that a converter is connected to its alternating-voltagenetwork "without the use of a separate winding transformer", or "fulltransformer" means that the converter is connected to itsalternating-voltage network in some way other than with the aid of aseparate winding transformer. A converter connected without the use of aseparate winding transformer may thus have its alternating-voltageterminals galvanically connected to the alternating-voltage network,directly or via an auto-transformer, and possibly via inductors forcurrent limitation. A converter connected without the use of a separatewinding transformer may alternatively have its alternating-voltageterminals connected to the alternating-voltage network via seriescapacitors.

Analogously, this application uses the concept "transformerless"connection for those cases where no transformer of any kind, thusneither a separate winding transformer nor an auto-transformer, is usedfor connection of a converter to its alternating-voltage network. Inthis case, thus, the converter may have its alternating-voltageterminals connected to the alternating-voltage network galvanically, orvia series capacitors.

BACKGROUND OF THE INVENTION

In an HVDC transmission, each one of the converters usually consists oftwo series-connected six-pulse bridges. Each bridge is connected to thealternating-voltage network via a separate winding transformer. Thetransformers of the bridges, or the valve windings of a commontransformer, are designed with different connections, usually star- anddelta-connections, in such a way that the alternating voltages of thebridges are phase shifted 30°. Hence, the converter is of twelve-pulsedesign. HVDC transmissions of this kind are amply described in theliterature, for example in Erich Uhlmann: "Power Transmission by DirectCurrent", Springer-Verlag Berlin Heidelberg New York 1975 (see, e.g.Fig. 2.7, page 15, or Fig. B.1, page 187).

Since the converter bridges are connected to the alternating-voltagenetwork via transformers, a possibility of technical-economicoptimization of the direct-voltage level and the dc level of thetransmission is provided. By connecting the converter bridges to thealternating-voltage network via separate winding transformers, galvanicseparation between the bridges and the alternating-voltage network isobtained. This means that, in the manner described above, two converterbridges may be direct-voltage series-connected. Hence, a higherresultant pulse number and a reduction of the harmonic content,theoretically an elimination of the lowest harmonics, may be obtained.In this way, the amount of filter equipment may be reduced. This isimportant since the cost of the filter equipment constitutes anessential part of the total cost of a typical HVDC installation. Thegalvanic separation also means that a converter cannot leak a directcurrent out into the alternating-voltage network. Current leakageincludes a risk of disturbances, such as transformer saturation.

The advantages of the type of converter station described above haveresulted in this type being practically universally prevailing in HVDCinstallation.

In the dissertation "HGU-Kurzkupplung ohne Transformatoren", byDipl.-Ing. Knut Gebhardt, Technische Hochschule Darmstadt, 1976/1977, ithas been proposed to connect the converters in a transformerless mannerin an HVDC back-to-back connection. This paper shows the above-describedconventional connection in Figure 1, page 4, and an example of atransformerless connection in Figure 2, page 5. At first glance, thetransformerless connection is economically advantageous since therelatively high cost of the convertor transformers is eliminated.However, the connection has several disadvantages, which have causedthis connection not to be used in practice to any significant extent.

First of all, the direct-voltage level of the installation is determinedby the voltage in the alternating-voltage networks. This means thatthere is no possibility of voltage and current optimization of the dclink and the converters. Secondly, a transformerless HVDC installationis limited to six-pulse operation of the converters. This entails theoccurrence of harmonics with low ordinal numbers (5 and 7), whichnecessitate considerably more costly equipment for harmonic filtering.Thirdly, in an installation of this kind, harmonic currents with ordinalnumbers which are odd multiples of 3, that is, harmonic currents of theordinal numbers 3, 9, 15, 21 . . . , are generated on the dc side of aconverter. These currents flow out into the alternating-voltage networkof the converter. In this network, the currents are of zero-sequencetype and give rise to considerable disadvantages in the form oftelecommunication disturbance and voltage distortion in the network. Inweak alternating-voltage networks, the voltage distortion becomes such aserious disadvantage that the transformerless connection cannot be usedwithout taking special steps.

However, it is, of course, possible to arrange a filter for harmoniccurrents of the ordinal numbers 3, 9, 15, 21 . . . , mentioned in thepreceding paragraph. Such a filter may thus be arranged on the ac sideof a converter. However, the filter will be large and expensive, and ithas proved to be difficult to avoid resonance effects between the filterand the alternating-voltage network. Alternatively, a filter for theharmonics just mentioned may be arranged in the form of a blockingfilter on the dc side of the converter. Also in this case, however, thedimensions and the costs of the filter equipment will be high, and aconsiderable risk of resonance effects arises. These facts have causedthe transformerless connection to be considered possible only inconnection with strong alternating-voltage network.

SUMMARY OF THE INVENTION

The present invention aims to provide an HVDC transmission of the kinddescribed in the introductory part of the description, which is simplerand less expensive than hitherto used transmissions. At the same time,by reducing or completely avoiding the above-described networkdisturbances on the alternating-voltage side in the form oftelecommunication interference and voltage distortion, the presentinvention may be used also in weak alternating-voltage networks.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail in the following withreference to the accompanying FIGS. 1-7.

FIG. 1 shows an HVDC transmission according to the invention designed asa back-to-back connection;

FIG. 2 shows in more detail the main circuits of one of the convertersin the back-to-back connection according to FIG. 1;

FIG. 3 shows an example of the embodiment of a phase of the harmonicfilter in the back-to-back connection according to FIG. 1;

FIGS. 4a, 4b, 4c and 4d show examples of the embodiment of thezero-sequence inductor in the back-to-back connection in FIG. 1;

FIG. 5 schematically shows an HVDC transmission consisting of twogeographically separated converter stations according to the invention,which are interconnected by a dc line;

FIG. 6 shows an HVDC transmission according to the invention, in whichthe two converters are connected to their alternating-voltage networksvia series capacitors.

FIG. 7a and FIG. 7b show two variants of a bipolar converter station inan HVDC transmission according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows, in the form of a single-line diagram, an embodiment of aback-to-back connection according to the invention. It is intended forcontrollable power transmission between two three-phase electric acpower networks N1 and N2. It consists of two controllable high-voltageconverters SR1 and SR2, arranged in a single converter station. The acleads VL1 of the converter SR1 are connected to the alternating-voltagenetwork N1 via a three-phase zero-sequence inductor connection IB,schematically shown in the figure. The inductor connection IB isdesigned such that it has a high impedance to zero-sequence currents anda low impedance to positive-sequence currents. The inductor connectionIB will be described in more detail below.

The ac leads VL2 of the converter SR2 are connected to thealternating-voltage network N2 via current-limiting inductors IL.Further, on its ac side, each converter has a schematically shownharmonic filter, F1 and F2, respectively. The dc terminals LL1P, LL1Nand LL2P, LL2N, respectively, of the converters are interconnected viaconductors L1 and L2. Conductors L1 and L2 consist of busbars or thelike in the converter station. In one of these conductors, a smoothinginductor IG for the direct current is arranged in conventional manner.

The inductor connection IB will effectively prevent zero-sequencecurrents of other than negligible magnitude from occurring on the acside of the converter SR1. This greatly reduces the disturbances,described above, which are caused by the presence of these currents inalternating-voltage networks. This makes possible the use of atransformerless HVDC transmission for connection not only to very strongalternating-voltage networks, but also to weaker networks.

The converters with their ac leads form together with the dc link L1-L2a continuous and branch-free current path for zero-sequence currents.The inductor connection IB prevents zero-sequence currents from arisingat one location in this current path, namely, on the ac side of theconverter SR1. No zero-sequence currents will therefore arise at otherlocations in this current path, that is, not on the ac side of theconverter SR2 either. Therefore, no special zero-sequence inductorconnection is arranged in the leads of the converter SR2.

FIG. 2 shows the main circuits of the converter SR1 in the back-to-backconnection according to FIG. 1. The converter is a converter of the samekind as is currently used for HVDC transmissions. It is a six-pulseline-commutated phase-angle controlled thyristor converter with thevalves TY1-TY6. Each valve consists of a plurality of series-connectedthyristors with parallel-connected damping circuits and overvoltageprotective means. The converter has the dc connections LL1P and LL1N andthe ac leads VL1R, VL1S and VL1T. Because of the presence smoothinginductor IG, shown in FIG. 1, arranged on the dc side of the converter,the function of the converter is that of a current-source converter.

As shown in FIG. 2, the zero-sequence inductor connection IB may consistof a three-phase inductor with an iron core IBC and three phase windingsIBR, IBS and IBT with the polarities shown in the figure and each oneconnected into one of the three leads of the converter. Forzero-sequence currents, the inductor has a very high impedance, whereasits impedance to positive-sequence currents is very low.

The converter SR2 is designed in the same way. As mentioned, however, itdoes not have the zero-sequence inductor connection but has instead thecurrent-limiting inductors IL, which replace the impedance of theconverter transformer existing in conventional HVDC transmissions. Theinductors may be air inductors. Each of the ac leads of the converterSR2 may include an inductor. Also, each inductor may have an inductanceadapted such that the inductors limit the valve currents in theconverter SR2 to harmless values for the types of faults occurring, forexample short circuits or ground faults. The inductor connection IB isdesigned with a leakage impedance of such a magnitude that it fulfilsthe corresponding function for the converter SR1.

FIG. 3 shows an example of the embodiment of a phase F1R in the filtercircuit F1 in the back-to-back connection according to FIG. 1. It isconnected to the ac conductor VL1R between the network N1 and theconverter SR1, preferably between the network and the inductorconnection IB. The filter circuit consists of double-tuned bandpassfilters for tones 5 and 7 (BP5/7), 11 and 13 (BP11/13) and 17 and 19(BP17/19). The filter also includes double-tuned high-pass filters fortones 24 and 42 (HP24/42) and for tones 30 and 36 (HP30/36). Thehigh-pass filters are suitably designed also provide sufficient dampingfor tones of ordinal numbers 47 and 49.

In the HVDC installation shown in FIG. 1, only one of theconverters--SR1--has a zero-sequence inductor connection in its acleads. The other converter--SR2--has current-limiting inductors in itsac leads. Alternatively, of course, zero-sequence inductor connectionsmay be arranged in the ac leads of both converters. Likewise, dependingon the impedances of networks and other components of the installations,current-limiting inductors may be arranged in the ac leads of bothconverters or, possibly, be completely omitted.

To achieve the desired function--a high impedance to zero-sequencecurrents and a low impedance to positive-sequence currents--it isimportant that the zero-sequence connection IB is designed such that agood magnetic coupling is obtained between the windings of the inductor.Further, it is important that a high degree of symmetry be obtained inthe inductor connection. FIGS. 4a-4d show examples of an embodiment of azero-sequence inductor connection according to the invention.

FIG. 4a shows the principle of a zero-sequence inductor connectionaccording to the invention. The windings IBR, IBS, IBT are arranged on acommon core IBC. The windings have the connections RN, SN, TN forconnection of that side of the windings facing the network andconnections RS, SS, TS for connection of that side of the windingsfacing the converter. Provided that the coupling and the symmetry arecomplete, positive-sequence currents provide ampere-turn balance in thecore. That is, no flux arises in the core. Also, the induced EMF and,hence, the impedance to positive-sequence currents become zero. Underthe same conditions, zero-sequence co-phasal currents in the threewindings, will cooperate and provide a flux in the core, that is, aninduced EMF and an impedance which is high, as long as the core does notbecome saturated.

To obtain as good a coupling between the windings as possible, it isadvantageous to arrange the windings one outside the other and to designthem as long as is allowed by the dimensions of the core. FIG. 4b showssuch an embodiment of the zero-sequence inductor. In this case, theinductor is, in principle, designed as a single-phase three-windingtransformer.

To obtain full symmetry, as shown in FIG. 4c, each one of the threewindings of the inductor connection may be divided into threeseries-connected sections. These sections are arranged in three groupssuch that each group contains three sections belonging to differentwindings and arranged one above the other. Also, the sections arearranged such that each winding has one section outermost in one group,in the center position in another group, and innermost in the thirdgroup. The winding IBR thus has the three sections IBR1, IBR2 and IBR3.Section IBR1 lies outermost in the uppermost group in the figure.Additionally, section IBR2 lies in the center position in the middlegroup. Furthermore, section IBR3 lies innermost in the lowermost group.

An alternative to the embodiment shown in FIG. 4c is to design thezero-sequence inductor connection as three series-connected units of thekind shown in FIG. 4b, wherein the radial location of the windings ispermuted between the three units in the same way as for the three groupsin FIG. 4c.

To obtain full symmetry, and to obtain ampere-turn balance in all partsof the core for positive-sequence currents, that is, to obtain thelowest possible positive-sequence impedance, the inductor connectionaccording to FIG. 4 is, in practice, designed in the manner shown inFIG. 4d. That is, the inductor connection is designed with each phasewinding divided into three series-connected sections. One of thesesections is arranged on each one of the three legs supporting windings.Also, the inductor connection is designed with the radial positions ofthe sections permuted between the three legs in the same way, inprinciple, as in the inductor connection shown in FIG. 4c.

FIG. 5 schematically shows an embodiment of an HVDC transmissionaccording to the invention consisting of two geographically separatedconverter stations with converters SR1 and SR2, which are interconnectedby means of a line DCL with the conductors LA and LB. The line mayconsist of an overhead line or a cable disposed in the ground or thewater, or of a combination thereof. Each station is designed in themanner shown in FIG. 1 and its components have the same designations asin FIG. 1.

In this case, however, contrary to the case with the installation shownin FIG. 1, each station includes a separate positive-sequence inductorconnection, IB1 and IB2, respectively, of the kind described above. Thisensures efficient blocking of the zero-sequence currents generated byeach converter. If only one of the converters included a zero-sequenceinductor, because of the normally significant line capacitance, anefficient blocking of the zero-sequence currents of the other converterwould not be obtained.

Further, in the transmission according to FIG. 5, each station has asmoothing inductor, IG1 and IG2, respectively.

FIG. 6 shows an embodiment of an HVDC transmission according to theinvention, which substantially corresponds to the embodiment shown inFIG. 5. In the embodiment shown in FIG. 6, the converters are connectedto their alternating-voltage networks via series capacitors C1 and C2,respectively. The series capacitors function as direct-voltage barriersand, hence, they make possible grounding of the direct-voltage side ofeach converter at one point. FIG. 6 shows how one pole of each converteris grounded, whereby the line between the converter stations may consistof one single conductor LA. The return of the direct current takes placethrough ground. FIG. 6 thus shows a monopolar HVDC transmission.

However, the use of series capacitors in the embodiment shown in FIG. 6also makes it possible to apply the invention to a bipolar HVDCtransmission. One of the two converter stations in such a transmissionis shown in FIG. 7a. It has two converters, SR1A and SR1B. Each of themis connected to the network N1 via a series capacitor, C1A and C1B,respectively, and a zero-sequence inductor, IB1A and IB1B, respectively.Filter equipment F1 for harmonic filtering, common to the converters, isconnected to the alternating-voltage side of the converters. Eachconverter is provided with a smoothing inductor, IG1A and IG1B,respectively. The station has a ground terminal GA, which is connectedto the station through a ground line LG. The converter SR1A has one ofits direct-voltage connections connected to one of the conductors LA ofthe transmission line via the inductor IG1A. The other direct-voltageconnection is connected to the ground line LG. The converter SR1B hasone of its direct-voltage connections connected to the otherconductor--LB--of the transmission line via the inductor IG1B and theother direct-voltage connection connected to the ground line LG.

Alternatively, as shown in FIG. 7b, a zero-sequence inductor IB1 whichis common to the two converters SR1A and SR1B may be arranged in thelead VL1, between the network N1 and the converters, which is common tothe converters.

In the foregoing, only such embodiments of the invention have beendescribed where all the converters included in the HVDC transmission areconnected to their respective alternating-voltage networks in atransformerless manner--either directly galvanically or via seriescapacitors. However, other embodiments are also feasible within thescope of the invention.

Thus, for example, a converter may be connected to itsalternating-voltage network via an auto-transformer. Such an embodimentoffers the advantages that the current and voltage levels of the dc linkand the converter may be chosen independently of the voltage of thealternating-voltage network. Therefore, the transmission may be used forpower transmission between alternating-voltage networks with differentvoltages. Also, the current-limiting inductors described above, forexample, IL shown in FIG. 1, may be superfluous. Furthermore, an on-loadtap changer of the transformer may be used to take up changes in theratio between the direct voltage of the converter and the voltage of thealternating-voltage network. This provides the possibility of operatingwith more optimal control angles of the converter from thereactive-power point of view. Since auto-transformers are considerablyless expensive than separate winding transformers, utilizing suchtransformers results in an important saving of costs as compared withconventional HVDC transmissions with separate winding transformers.

The converters at both ends of a dc link may be connected to theiralternating-voltage networks via auto-transformers. Alternatively onlythe converter at one end of the link may be connected to analternating-voltage network via an auto-transformer. In the latter case,the converter at the other end of the link is either transformerless orconnected to its alternating-voltage network via a separate windingtransformer.

As mentioned above, one of the converters of a transmission may beconnected to its alternating-voltage network via a separate windingtransformer. The converter at the other end of the link may then eitherbe transformerless or connected via an auto-transformer. In this case,the tap-changer of the separate winding transformer may be utilized totake up variations in the voltage of the alternating-voltage network.Also, the reactive-power consumption may thereby be kept low.Additionally, the dc link and the converters may be optimizedindependently of the voltage of the alternating-voltage network in thestations provided with transformers. Furthermore, where the converter isconnected via a separate winding transformer, no mutual inductor isneeded for blocking the effect of the link on the alternating-voltagenetwork.

In those embodiments of an HVDC transmission according to the presentinvention in which a converter is not connected to itsalternating-voltage network via a transformer provided with atap-changer, variations in the ratio between the voltage of thealternating-voltage network and the voltage of the dc link must be takenup by changes of the control angle of the converter. This entails anundesirable increase of the reactive-power consumption of the converterand/or of the variations in this consumption. If considered necessary,these disadvantages may be counteracted or eliminated by providing thealternating-voltage side of the converter with known controllablereactive-power means, such as capacitors, inductors, or, for example, acombination of switchable capacitors and phase-angle controlledinductors.

In those cases where an HVDC transmission according to the invention hasa transformerless connection of the converter at one end of the dc linkand a converter connection via a transformer provided with a tap-changerat the other end of the link, it is suitable to allow the formerconverter to operate with a fixed control angle selected to minimize thereactive-power consumption, whereby variations in the ratio between thevoltages of the alternating-voltage networks are taken up by thetap-changer.

It has been found that the negative influence of HVDC transmission onthe alternating-voltage network may be maintained especially low if aconnection without a separate winding transformer according to theinvention is provided with a control system that controls each of thetwo halves of a six-pulse converter individually in such a way thatdirect currents in the two dc leads of the converter are alwaysmaintained as equal as possible.

An HVDC transmission between two or more geographically separatedconverter stations, which are interconnected by means of overhead linesor cables, may contain two or more converters connected without the useof separate winding transformers. In this case, as shown in FIG. 5, eachconverter or converter station connected without the use of a separatewinding transformer may have a zero-sequence inductor connection on itsac side. Alternatively, only one of, or certain ones of, the converterstations may be provided with such inductor connections. The otherconverter/stations connected without a separate winding transformer maythen be provided with some other type of means for blockingzero-sequence currents, for example with a zero-sequence inductorconnection on the dc side.

I claim:
 1. An HVDC transmission, comprising:at least two converters,wherein each converter is adapted for connection between a three-phasealternating-voltage network wherein at least a first converter has acleads for connection of the first converter to the alternating-voltagenetwork without the use of a separate winding transformer; a dc linkcommon to the converters a zero-sequence inductor connection arranged inthe ac leads of the first converter and designed such that thezero-sequence inductor connection exhibits a high impedance tozero-sequence currents and a low impedance to positive-sequencecurrents.
 2. An HVDC transmission according to claim 1, wherein at leastthe first converter is a current-source line-commutated converter.
 3. AnHVDC transmission according to claim 1, wherein said HVDC transmissionis a back-to-back connection including two converters which aredc-connected to each other, and wherein each of the two converters isconnected to a separate alternating-voltage network without utilizing aseparate winding transformer, and wherein only one of the converters isprovided with a zero-sequence inductor connection arranged in the acleads.
 4. An HVDC transmission according to claim 1, further comprisinga dc line connecting said at least two converters, wherein said at leasttwo converters are geographically separated from each other.
 5. An HVDCaccording to claim 1, further comprising series capacitors arranged inthe ac leads of the converter, wherein at least one of the converters isconnected to the alternating-voltage network without the use of theseparate winding transformer and via said series capacitors.
 6. An HVDCtransmission according to claim 5, further comprising a converterstation including two converters connected to an alternating-voltagenetwork, each of said converters being dc-connected between one of twopole conductors said HVDC tramnission and a common ground line, whereinsaid HVDC transmission is designed as a bipolar transmission and whereineach of the two converters is connected to the alternating-voltagenetwork via the series capacitors arranged in the ac leads of theconverter.
 7. An HVDC transmission according to claim 6, wherein each ofthe two converters of the station includes the separate zero-sequenceinductor connection.
 8. An HVDC transmission according to claim 6,wherein the zero-sequence inductor connection common to the twoconverters of the station is arranged between the converters and thealternating-voltage network.
 9. An HVDC transmission according to claim1, further comprising current-limiting reactors are arranged in the acleads to at least one of the converters.
 10. An HVDC transmissionaccording to claim 1, wherein each converter includes analternating-voltage side, each alternating-voltage side being providedwith harmonic filter circuits adapted for filtering of tones of theordinal numbers 6 m±1, where m is a positive integer, and wherein thefilter circuits for the converters which are connected without anyseparate winding transformer comprise filters for tones of the ordinalnumbers 5 and
 7. 11. An HVDC transmission according to claim 1, whereinthe zero-sequence inductor connection includes an iron core and threewindings arranged on the core, wherein each one of the windings isarranged in one of the three ac leads to one of the converters.
 12. AnHVDC transmission according to claim 11, wherein the windings arearranged radially one outside the other.
 13. An HVDC transmissionaccording to claim 11, wherein each winding includes three windingsections electrically series connected to each other, wherein thesections are arranged in three winding groups with each group comprisingone section from each one of the three windings, wherein the threesections in each group are arranged radially one outside the other, andwherein the radial positions of the sections are permutated between thegroups.
 14. An HVDC transmission according to claim 13, wherein thewinding groups are arranged one after the other along one core leg. 15.An HVDC transmission according to claim 13, wherein the iron coreincludes five core legs, and wherein the windings are arranged on threeof the core legs with one of the three winding groups on each one of thethree legs provided with a winding.